How to Pass data to the current boost meta state machine(MSM) substate

问题

In the following example, while the current execution is still in substate1, I want to pass data to substate1 continuously and then send Event3 or Event1 based on the data. Looks like MSM supports only sending events using (process_event()), but I am not sure how to send data continuously to the current state.

#include <iostream>
#include <boost/msm/back/state_machine.hpp>
#include <boost/msm/front/state_machine_def.hpp>
#include <boost/msm/front/functor_row.hpp>

namespace {
    namespace msm = boost::msm;
    namespace msmf = boost::msm::front;
    namespace mpl = boost::mpl;

    // ----- Events
    struct Event1 {};
    struct Event2 {};
    struct Event3 {};

    // ----- State machine
    struct OuterSm_:msmf::state_machine_def<OuterSm_>
    {
        struct State1_:msmf::state_machine_def<State1_>
        {
            template <class Event,class Fsm>
            void on_entry(Event const&, Fsm&) const {
                BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, OuterSm_>::value));
                std::cout << "State1::on_entry()" << std::endl;
            }
            template <class Event,class Fsm>
            void on_exit(Event const&, Fsm&) const {
                BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, OuterSm_>::value));
                std::cout << "State1::on_exit()" << std::endl;
            }

            struct SubState1:msmf::state<> {
                template <class Event,class Fsm>
                void on_entry(Event const&, Fsm&) const {
                    BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, State1_>::value));
                    std::cout << "SubState1::on_entry()" << std::endl;
                }
                template <class Event,class Fsm>
                void on_exit(Event const&, Fsm&) const {
                    BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, State1_>::value));
                    std::cout << "SubState1::on_exit()" << std::endl;
                }
            };
            struct SubState2:msmf::state<> {
                template <class Event,class Fsm>
                void on_entry(Event const&, Fsm&) const {
                    BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, State1_>::value));
                    std::cout << "SubState2::on_entry()" << std::endl;
                }
                template <class Event,class Fsm>
                void on_exit(Event const&, Fsm&) const {
                    BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, State1_>::value));
                    std::cout << "SubState2::on_exit()" << std::endl;
                }
            };

            // Set initial state
            typedef mpl::vector<SubState1> initial_state;
            // Transition table
            struct transition_table:mpl::vector<
                //          Start      Event   Next       Action      Guard
                msmf::Row < SubState1, Event2, SubState2, msmf::none, msmf::none >,
                msmf::Row < SubState2, Event3, SubState1, msmf::none, msmf::none >
                > {};
        };
        struct State2:msmf::state<>
        {
            template <class Event,class Fsm>
            void on_entry(Event const&, Fsm&) const {
                BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, OuterSm_>::value));
                std::cout << "State2::on_entry()" << std::endl;
            }
            template <class Event,class Fsm>
            void on_exit(Event const&, Fsm&) const {
                BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, OuterSm_>::value));
                std::cout << "State2::on_exit()" << std::endl;
            }
        };

        typedef msm::back::state_machine<State1_> State1;

        // Set initial state
        typedef State1 initial_state;
        // Transition table
        struct transition_table:mpl::vector<
            //          Start   Event   Next    Action      Guard
            msmf::Row < State1, Event1, State2, msmf::none, msmf::none >
        > {};
    };

    // Pick a back-end
    typedef msm::back::state_machine<OuterSm_> Osm;

    void test()
    {        
        Osm osm;
        osm.start(); 

        std::cout << "> Send Event2()" << std::endl;
        osm.process_event(Event2());
        std::cout << "> Send Event1()" << std::endl;
        osm.process_event(Event1());
    }
}

int main()
{
    test();
    return 0;
}

Output:

State1::on_entry()
SubState1::on_entry()
> Send Event2()
SubState1::on_exit()
SubState2::on_entry()
> Send Event1()
SubState2::on_exit()
State1::on_exit()
State2::on_entry()

回答1:

You can use get_state to retrieve a state from the state machine and then you can call an arbitrary method on that state, e.g. sendData() in the following modification of your example (live example):

#include <iostream>
#include <boost/msm/back/state_machine.hpp>
#include <boost/msm/front/state_machine_def.hpp>
#include <boost/msm/front/functor_row.hpp>

namespace {
    namespace msm = boost::msm;
    namespace msmf = boost::msm::front;
    namespace mpl = boost::mpl;

    // ----- Events
    struct Event1 {};
    struct Event2 {};
    struct Event3 {};

    // ----- State machine
    struct OuterSm_:msmf::state_machine_def<OuterSm_>
    {
        struct State1_:msmf::state_machine_def<State1_>
        {
            template <class Event,class Fsm>
            void on_entry(Event const&, Fsm&) const {
                BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, OuterSm_>::value));
                std::cout << "State1::on_entry()" << std::endl;
            }
            template <class Event,class Fsm>
            void on_exit(Event const&, Fsm&) const {
                BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, OuterSm_>::value));
                std::cout << "State1::on_exit()" << std::endl;
            }

            struct SubState1:msmf::state<> {
                template <class Event,class Fsm>
                void on_entry(Event const&, Fsm&) const {
                    BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, State1_>::value));
                    std::cout << "SubState1::on_entry()" << std::endl;
                }
                template <class Event,class Fsm>
                void on_exit(Event const&, Fsm&) const {
                    BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, State1_>::value));
                    std::cout << "SubState1::on_exit()" << std::endl;
                }

                void sendData(const std::string& data)
                {
                    std::cout << "data received in Substate1:" << data << std::endl;
                }
            };
            struct SubState2:msmf::state<> {
                template <class Event,class Fsm>
                void on_entry(Event const&, Fsm&) const {
                    BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, State1_>::value));
                    std::cout << "SubState2::on_entry()" << std::endl;
                }
                template <class Event,class Fsm>
                void on_exit(Event const&, Fsm&) const {
                    BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, State1_>::value));
                    std::cout << "SubState2::on_exit()" << std::endl;
                }
            };

            // Set initial state
            typedef mpl::vector<SubState1> initial_state;
            // Transition table
            struct transition_table:mpl::vector<
                //          Start      Event   Next       Action      Guard
                msmf::Row < SubState1, Event2, SubState2, msmf::none, msmf::none >,
                msmf::Row < SubState2, Event3, SubState1, msmf::none, msmf::none >
                > {};
        };
        struct State2:msmf::state<>
        {
            template <class Event,class Fsm>
            void on_entry(Event const&, Fsm&) const {
                BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, OuterSm_>::value));
                std::cout << "State2::on_entry()" << std::endl;
            }
            template <class Event,class Fsm>
            void on_exit(Event const&, Fsm&) const {
                BOOST_STATIC_ASSERT((boost::is_convertible<Fsm, OuterSm_>::value));
                std::cout << "State2::on_exit()" << std::endl;
            }
        };

        typedef msm::back::state_machine<State1_> State1;

        // Set initial state
        typedef State1 initial_state;
        // Transition table
        struct transition_table:mpl::vector<
            //          Start   Event   Next    Action      Guard
            msmf::Row < State1, Event1, State2, msmf::none, msmf::none >
        > {};
    };

    // Pick a back-end
    typedef msm::back::state_machine<OuterSm_> Osm;

    void test()
    {        
        Osm osm;
        osm.start(); 

        Osm::State1& state1 = osm.get_state<Osm::State1&>();
        Osm::State1::SubState1& substate1 = state1.get_state<Osm::State1::SubState1&>();
        substate1.sendData("hello from outside");

        std::cout << "> Send Event2()" << std::endl;
        osm.process_event(Event2());
        std::cout << "> Send Event1()" << std::endl;
        osm.process_event(Event1());
    }
}

int main()
{
    test();
    return 0;
}

Output:

State1::on_entry()
SubState1::on_entry()
data received in Substate1:hello from outside
> Send Event2()
SubState1::on_exit()
SubState2::on_entry()
> Send Event1()
SubState2::on_exit()
State1::on_exit()
State2::on_entry()

来源：https://stackoverflow.com/questions/32447638/how-to-pass-data-to-the-current-boost-meta-state-machinemsm-substate